The catalytic hydrogenation of anilines to give the corresponding symmetrical dicycloaliphatic amines in the presence of noble metal catalysts is known. There exist only a few publications on the hydrogenation of anilines in the presence of noble metal catalysts at low pressure.
EP-A 0 324 983 and EP-A 0 324 984, herein incorporated by reference in their entirety, describe processes using basically modified catalysts which comprise both Ru and Pd. Whereas, in EP-A 0 324 983, considerable pressure is employed and only low dicyclohexylamine contents are achieved, EP-A 0 324 984 describes a low-pressure process having a high dicyclohexylamine yield. However, the catalysts having approximately 0.1-0.2 kg of starting material per liter of catalyst have only a low space velocity.
FR 1,530,477, herein incorporated by reference in its entirety, describes a low-pressure process in which aniline is reacted with large amounts of ammonia in a hydrogen stream in the presence of Pd supported catalyst at temperatures between 175 and 190.degree. C. The product comprises large amounts of dicyclohexylamine.
EP-A 0 560 127, herein incorporated by reference in its entirety, describes a low-pressure process in which aniline is reacted in the presence of base-modified Ru--Pd supported catalysts. The catalysts have only a very low space velocity.
EP-A 0 208 933, herein incorporated by reference in its entirety, describes Rh catalysts on supports modified by Cr--Mn salts. The catalysts were developed for the dehydrogenation of precursors for o-phenylphenol synthesis at high temperatures.
EP-A 0 535 482, herein incorporated by reference in its entirety, likewise describes heat-stable Rh catalysts on supports modified by Cr--Mn salts for preparing o-phenylphenol, the catalysts comprising other nobel metals in addition to the Rh.
The Rh catalysts can be used for the dehydrogenation in thermostatic steady-state catalyst beds at low pressures and temperatures between 300 and 400.degree. C.
U.S. Pat. No. 5,360,934, herein incorporated by reference in its entirety, discloses a process for hydrogenating aromatic amines in the presence of a rhodium catalyst which is applied to a support of .kappa.-, .theta.- or .delta.-Al.sub.2 O.sub.3. U.S. Pat. No. 4,960,941, herein incorporated by reference in its entirety, likewise discloses a process for hydrogenating aromatic amines in the presence of a rhodium catalyst. In this case, the rhodium catalyst is applied to a TiO.sub.2 support. In both cases, the hydrogenation is carried out in the liquid phase under pressure.
Applications on the low-pressure hydrogenation of anilines using catalysts which comprise Rh as noble metal component are not known, although the literature reports that Rh catalysts are said to be suitable for low-pressure hydrogenations of anilines (P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, 1967, pp. 331-363; P. N. Rylander, Hydrogenation Methods, Academic Press, 1985, pp. 123-133).
A prejudice against developing an Rh catalyst for producing dicyclohexylamines at low pressure was generated by a work published a few years ago on the gas phase hydrogenation of aniline in the presence of Rh on .gamma.-Al.sub.2 O.sub.3. Although increasing conversion rates were achieved with increasing Rh content of the catalyst at 1 atm and 200.degree. C., the cyclohexylamine selectivity, at approximately 20%, is very low, regardless of the conversion rate. The dicyclohexylamine selectivity even decreases with increasing Rh content, and thus with increasing conversion rate, from 40% to 20%, so that predominantly unwanted products were obtained. (V. Vishwanathan, S. Narayanan, J. Chem. Soc., Chem. Commun., 1990, 78-80).
The publication suggests that rhodium catalysts are unsuitable, at low pressures in the gas phase, for the industrial hydrogenations of anilines to give cyclohexylamines and dicyclohexylamines.
The object underlying the present invention was to find a selective and high-space-velocity low-pressure process for the hydrogenation of aromatic amines, preferably of anilines, to give dicycloaliphatic amines, preferably dicyclohexylamines.
Surprisingly, it has been found that catalysts which comprise Rh on specially treated support materials are potent catalysts for implementing a process for the low-pressure hydrogenation of aromatic amines to give dicycloaliphatic amines.